A sentence is a set of words that function as a unit to convey a coherent idea. A sentence must have a subject and a verb. For example, “I run,” is a complete sentence even though it only contains two words.

There are also many other components we could include in a sentence. There are direct and indirect objects, which will be discussed further in the next section. We also have adjectives, adverbs, prepositional phrases, participial phrases, and infinitive phrases. Some of these will be discussed in later sections as well. For now, let’s focus on three different types of sentences. 

• A simple sentence is a sentence that contains only one clause. A clause is a group of words that consists of a subject and verb. In simple sentences it is common to find the subject before the verb: “The boy fell off his bike this afternoon.” There is only one clause (one verb and one subject) in this sentence, so it is a simple sentence. The modifiers, or words that add additional information about the subject or verb (his bike, this afternoon), do not affect the classification of the sentence. 
• Compound sentences have two equivalent clauses connected by a coordinating conjunction. A coordinating conjunction joins words, phrases, or clauses that are equal. In other words, it coordinates the relationship between two complete sentences, each of which could stand alone. Here are common coordinating conjunctions you will recognize: for, and, nor, yet, but, or. Here is an example: “The boy fell off his bike, and he went to the hospital.” If we remove and from the sentence, both clauses could stand alone as complete sentences. 
• A complex sentence has two parts, a main clause (also called an independent clause) and a dependent clause. A main clause can stand alone as a complete sentence. A dependent clause cannot stand alone as a complete sentence since it depends on the main clause for it to make sense. The order of clauses is more flexible in a complex sentence. Study this example: “Although the boy broke his arm, he still wanted to ride his bike.” If one were to only write, “Although the boy broke his arm,” it would not make sense. This is the dependent clause. The main clause is the second one: “he still wanted to ride his bike when he got home.” 

Resources:

• Color coded examples of sentence structure to visualize the different types of sentences. There is also a an excellent set of brief videos at the bottom of the page to explain sentence structure and components.
• More examples that include visuals of overall sentence structure.